The present invention relates to liquid dielectric capacitors and more particularly to a protective circuit for such capacitors.
Liquid dielectric capacitors, such as those commonly used in ballasts for fluorescent light fixtures, air conditioners, and the like, are subject to failure due to a "thermal runaway" condition wherein the temperature and pressure within the capacitor casing rise rapidly and uncontrollably. The pressure buildup may cause a rupture in a seam of the capacitor casing thereby permitting the liquid dielectric to flow from the casing into contact with other electrical components. This may cause damage to the electrical fixture or anything else onto which the dielectric may spill.
It has been observed that the thermal runaway condition can be halted if the capacitor is deenergized sufficiently early once the thermal runaway process begins. There is a point, however, at which the pressure buildup within the capacitor will continue regardless of whether or not the capacitor is thereafter deenergized. Accordingly, it is extremely important to deactivate the capacitor as soon as possible after thermal runaway begins to prevent the point of no return from being reached.
Heretofore, various schemes have been suggested to protect liquid dielectric capacitors from the effects of an excess energy buildup. Such prior art devices are disclosed, for example, in U.S. Pat. Nos. 1,578,563; 2,572,901; 2,896,049; 3,246,205; 3,248,617; and, 3,377,510.
In the main, the prior art devices rely on the pressure buildup within the capacitor can to cause swelling of the can cover. As the swelling progresses, it causes a rupture in an electrical lead running between the capacitance means and terminals on the can cover. To this end, a portion of the electrical lead is held fixed while the cover bulges so as to impart a stress to the lead. In addition, the lead may be preweakened or otherwise adapted to rupture upon bulging of the cover.
The principal object of the present invention is to provide an improved protective circuit for a liquid dielectric capacitor which responds more quickly and efficiently to pressure buildups within the capacitor can than was heretofore possible with existing devices.
A further object of the invention is to provide a protective circuit for such liquid dielectric capacitors wherein the possibility of accidental reconnection of the electrical leads, once disconnected, is avoided.
A still further object of the present invention is to provide such a protective circuit which may readily and easily be incorporated into conventional liquid dielectric capacitor cans with a minimum of modification or redesign of existing components.